Intraseasonal Modulation of South American Summer Precipitation

Intraseasonal variation of convection over South America is examined using singular spectrum analysis (SSA). The dipole convection pattern with centers of action over the SACZ and the subtropical plains is modulated by modes of different timescales. Both oscillatory modes with periods of 36–40 days (mode 40) and 22–28 days (mode 22) influence convection over the SACZ with the faster mode (mode 22) leading the variability over the subtropical plains. Mode 40 is related to the Madden–Julian oscillation. Outgoing longwave radiation anomalies (OLRA) propagate eastward from the western Pacific to the central Pacific with a period of 40–48 days. The 200-hPa streamfunction composites show a wavenumber 1 structure in the Tropics and a wave train propagating downstream from the convective area in the tropical Pacific. The development of the dipole pattern is also contributed by mode 22, which features the meridional propagation of OLRA over South America from midlatitudes to the Tropics. The streamfunction composites for mode 22 show a wave train extending from the central Pacific eastward to about 60°S and curving toward the northeast over South America. When the SACZ is enhanced, these two modes become meridionally aligned locally. Such episodes are characterized by a wave train propagating northeastward from southern South America toward the Tropics. Composites based on the SSA decomposition indicate that the dominant periods of variability (22 and 40 days) cancel out over the Pacific region and reinforce each other over the SACZ for times close to onset of the enhanced SACZ episodes.